System for frequency conversion and comparison of frequencies



A. KAHN 2,401,647

OF FREQUENCIES June 4, 1946.

SYSTEM FOR FREQUENCY CONVERSION AND COMPARISON Original Filed Aug. 28, 1941 MMM -|NvENToR vom@ www@ Wwwbkb Patented June 4, 1946 SYSTEM FOR FREQUENCY CONVERSION AND COMPARISON F FREQUENCIES Alfred Kahn, Hollis, N. Y., assigner to Radio Corporation of America, a corporation of Dela- Ware Original application August 28, 1941, Serial No. 408,633, now Patent No. 2,349,501, dated May 23, 1944. Divided and this application .September 28, 1943, Serial No. 504,114

`This invention relates to synchronizing systems and has particular reference to a device for synchronizing a given frequency `source with a known standard frequency generator. 'Ihe invention also comprehends means for multiplying or dividing either of two frequencies so that their derivatives may be compared and synchronized one with the other.

This application is a division of my copending application Serial No. 408,633, which was filed August 28, 1941, Patent No. 2,349,501, dated May 23,1944.

In multiplex telegraph systems, it is common practice to maintain synchronism between local and remote apparatus by means of tuning forks and phase correction devices. Usually, the transmitting stationis provided with a tuning fork control for the distributor speed. It is also a common practice to generate an alternating current, the frequency of which is maintained substantially constant by means 'of a tuning fork. When telegraph signals are transmitted through a multiplex distributor, the speed of which is thus controlled, it is possible, by well-known means, to maintain the speed of a distributor at a receiving station in synchronism with the received signals.

In some of the more extended communications systems, it has been -found that a number of different tuning `forks are necessary in order to ,provide all the frequency controls that are necessary. It is desirable to check the frequency of these tuning forks from time to time with an even better standard of frequency. Such a, standard is one which is quite costly to build and maintain and, therefore, precautions =are taken to operate the standard `itself `under ideal conditions such as would minimize the frequency drift therein.

It is an object of this invention to provide convenient means for checking certain practically operable frequency control `devices with the ,fre-

2 Claims. (Cl. 2504-36) be of the type which embodies a gaseous glow tube rotating on the shaft of a synchronous motor. The speed of the motor may be determined by the frequency of the alternating current derived from the tuning fork. On the other hand, the flashing of the 4glow tube may be made by means of short, sharp impulses derived from a laboratory frequency standard.

My invention will now b e described in more detail, reference being made `to the accompanying drawing, the sole figure of which represents diagrammatically a typical embodiment.

Referring to .the drawing, I show a pair of input terminals l on which may be impressed, for exampleanalternating current of 480 cycles per second, representing thewoutput from a tuning fork generator the frequency of which is to be compared with that of a llaboratory standard.

The 480-cycle frequency is fed across transformer 3 to the input circuit ,of a discharge tube 5. This tube is self-biased by means of cathode resistor R2, which is shunted by capacitor Cl. The input circuit includes a grid resistor RI and the secondary of transformer 3,

A limiter stage comprising discharge tube 'I is controlled by output from tube 5. The control potential is developed across resistor R3 the anode-connected end of which is coupledthrough capacitor C2 and resistor R5 to the control grid of tube 1. Grid leak resistor R4 is connected to the grounded cathode of tube 1 The output from tube 'I is a square wave which serves to synchronize a multivibrator comprising discharge tubes 9 4and Il. The circuit constants of these tubes are preferably selected to produce a LlO-cycle frequency, thus `dividing the 40o-cycle synchronizing frequency by 12. For tuning the multivibrator `the values of resistors R6, R8, R9, RIU, and RI If, and of capacitorsC3 and C4 may be determined empirically. ,Careishould be taken that resistors RIU `and Rl I are not subject to ohmic variations due to temperature or humidity. The values `ofcapacitors C3 and C4 should also be substantiallyv independent of temperature Aand humidity variations. ,A .critical tuning adjustment of the multivibrator is made by fixing the value of resistor R6 so that vthe multivibrator will keepin step with `the twelfth sub-harmonic of the 18o-cycle voutput from tube 1. The anode voltages in tubes 1, `9, and Il are adjusted by means of resistorR'I.

The grid of tube Il is coupled to the `anode of tube 9, and the gridof tubeis coupled to Vtheanode `of tube Il. This cross-coupling of the input and output `electrodes in lthe two tubes provides 3 l the well-known multivibrator action which is desired in order to produce a llcycle output which is synchronized with the 12th sub-harmonic of the ISO-cycle tuning-fork generator.

The square-wave output from tube vI-is Vapplied across resistor RI2 and capacitor C5 tothe grid lof an amplifier tube I3. This tube is self-biased f by means of a cathode resistor RI3. The resonant circuit consisting of inductance I and capacitor C6 is connected between thegrid of tube I3 and ground, thus providing a smoothing action Y which results in the output from tube I3 of a 40- capacitor C1, the latter being connected between.

the anode of tube I3 and ground. i l

The secondary of transformer II is connected to input terminals of a power amplifier I9, which may be of a type commonly used toddeliver freduency-controlled power to one or 'more synchronous motors. Such motors are, in turn, used to'drive telegraph distributors, and the like. In this case, however, the operation 0f my invention is best explained by showing output leads from the amplifier I9 connected to a synchronous motor 2|, the shaft of which carries a small neon lamp 23. The excitation of this lamp is made intermittent by means of short sharp pulses at a frequency of 20 cycles per second, which corresponds with the nominal shaft rotation speed of the motor 2I. The'derivation of the pulses for excitation of the lamp 23 will now be explained. AAssuming that a 100G-cycle source is available for reference purposes and that this source is maintained within very close limits of variation, accordingv to vlaboratory standardsgthen output energyfrom such a source will be applied to input terminals Zand impressed across transformer 4 upon Ya rst stage input circuit' of a twin triode discharge tube 6. This input circuit is tuned by `a capacitor C8 in shunt with the secondary of transformer 4. Resistor RI5 also forms part of the same input circuit.

`The rst and second discharge zones of tube 6 are controlled respectively by grids a and b. The output 'circuits of the, first and second zones are cascaded.A Grid b is in circuit with resistor RI'I and withv the secondary of transformer 8. The primary'of this transformer is tuned by `capacitor CID and is fed with an alternating potential developed across resistor RIB which is disposed in the output 'circuit of the first discharge zone in tube. y CapacitorCS isolates the directy current anode potential in the firstdischarge zone of tube 6 from the grounded primary of transformer 8.

Discharge tubel I0 has input and output circuits which aresimilar to those of tube 'I. Its input `circuit includes resistors RIB and R20, and the junction between these resistors is lcoupled across capacitor CII'to the anode of the second discharge zone in tube 6. A load resistor Rl8 isv to produce frequency division by 10Aof the 1000- r cycle synchronizing frequency. Theoperation of 4 o this multivibrator will, therefore, be understood without further description, except to point out that the values of resistors R25 and R21 are suitablydifferentiated for thefpurpose of providing such compensation for unbalance asv willcause the multivibrator to be maintained in step with an even sub-harmonic (the tenth) of the 100G-cycle control frequency. Resistor R22 determines the anode potentialsapplied in tubes I0, I2, and I4.

Output impulses from the multivibrator described in the preceding paragraph are fed through capacitor C` I 4 and resistor R28 to the input circuit of a succeeding stage comprising a gaseous discharge tube I6. To ignite this tube, such impulses are applied negatively to the cathode. The control grid is connected to the cathode through resistors R29 and R3I, the junction between these two resistors being grounded. Anode potential is supplied to the tube from a direct current source `I8. through resistor R34. In order to adjust the ignition and extinction potentials, resistor R3I is constituted as a variable poten# tiometer, the movable tap on which is connected to one end of resistor R33 for feeding a suitable Y positive potential thereto with respect to ground. An appreciable time is required for capacitor CI5 to be charged through resistor R34.v During this time, the anode potential in tube I6 builds up gradually to a Value such that the next succeeding negative impulse applied to the cathode will produce an ionization discharge. This charge, however, is of only momentary duration since the sudden discharge of capacitor CI5 again reduces the anode potential to a value below that which will sustain ionization. lThe short sharp discharges in tube I6 result in the delivery of momentary excitation potentials to a neon lamp4 23 used for purposes of indicatingl the' frequency drift.

The values ofA resistor R34 and capacitor CI5 are so determined, therefore, that the tube I6 acts as a frequency vdivider and its periodicity is preferably such as to divide the control impulses u by 5, thus producing an output of 20 vcycles per @i5 second, whereas the Aoutput from the multivibrator 4tubes I2, I4 was stated to be l10() cycles per second. Y Y

One electrode of theY lamp 23 is connected through a collector ring and brush andV through f Y tor 2l. K The flashes appear through theftrans-l lucent disc 22,. The disc 22 may be graduated in degrees, or, asanV alternative, Amay be graduated in terms of percentage deviation ofthe vfrequency of the tuning fork source with respect toal corresponding fixed standard of thesa-mc frequency;

It will be clear fromthe above description that the ashing of the lamp23 once per revolution around the scale of the disc 22 can be observedV as an indication of frequency drift'with respect to the LISO-cycle fork frequency in'relationrto the 100G-cycle laboratary standard. The motor 2I is mounted on a Achassis'behind a stationary calibrated translucent disc 22.? `A neon lamp 23 mounted' on the motor ,shaft is excited by pulses generated from the rlllo-cyclej'standard and is viewed through the aforementioned disc, which may be calibrated in degrees. With the motor operating at a shaft speed of 20 R. P. S. and the neon lamp flashing once per revoluticn, a convenient means is thus aiforded for reading fork generator accuracy directly from the calibrated dial 22. Furthermore, a short time observationwill indicate to the operator whether the working standard is slow or fast and to what degree. The change in fork speed caused by adjustmentl can ybe observed immediately so that a more accurate setting can be made in a brief space of time. The last statement refers to normal operating adjustments made by means of a fork drive potentiometer (not shown) and does not, of course, include major thermostat readjustments which may be found necessary from time to time.

The advantage to be derived from my improved frequency drift indicator will be better understood from the following discussion of the theory of operation.

By observing the amount of drift in degrees for a given period of time, the speed difference in parts in 100,000 can be readily determined. For example, in one second, the motor rotation is 20 revolutions, during which time the neon lamp 23 sweeps through '7200 degrees, provided the fork generator frequency is exactly what it should be. The relation between a: degrees of drift per second and the frequency error in parts per 100,000 is shown by the equation L 720c*c,000

7i2=parts in 100,000

It is perhaps preferable to divide the circumference of the scale on the disc 22 in 50 divisions. Each division Will then be 7.2", so that the frei quency drift in parts in 100,000 will correspond with the number of scale divisions through which the neon flash drifts in 100 seconds.

f The operator by observing the direction of drift of the neon flash can tell whether the 480- cycle fork generator is slow or fast. If the drift is clockwise, this standard is fast; if counterclockwise, it is slow.

The foregoing description may suggest modications of my invention, particularly to those skilled in the art, but such modifications should be considered ccmprehended Within the scope of the invention itself.

I claim: Y

1. In combinationwith a source of recurring Wave trains, a frequency divider comprising a gaseous discharge tube having a cathode, an anode, and a control electrode, means for applying potential variations from said source to one electrode of said tube, a direct current source having its positive terminal connected. through a resistor to the anode of said tube and through a tap on a voltage divider to the cathode thereof, a resistive connection from the negative terminal of said direct current source to said control electrode, another connection from the negative terminal of said direct current source to the end of said voltage divider remote from said cathode, a capacitor connected between the anode of said tube'and the negative terminal of said direct current source, said resistor and said capacitor constituting a time constant circuit whereby the ignition voltage in said tube is gradually built up during the occurrence of a predetermined number of waves in one of said trains, and means for so adjusting the tap on said voltage divider that said tube is caused to strike only in response to the nal Wave of eachA recurring wave train.

2. Apparatus for deriving a succession of discrete impulses of relatively brief duration from a source of sine waves of a frequency harmonically related to the periodicity of said impulses, said apparatus comprising means for limiting the amplitude of said sine waves to produce relatively square waves, means including a multivibrator for generating oscillations at a frequency which is controlled by said square Waves, a direct current source, a capacitor and resistor for storing electrical energy from said source at a constant rate during the generation of recurring oscillatory trains, each train possessing a predetermined number of waves, a gaseous discharge device through which said energy is caused to be suddenly released under control of the terminating oscillation in each said train, and adjustable potentiometric means having impedance connec-V tions across the terminals of said source and to the cathode of said gaseous discharge device for applying a suitable marginal voltage across the discharge path in the latter, whereby said terminating oscillation is rendered effective to fix the sub-harmonic frequency of said discrete impulses in relation to the frequency of said sine wave source.

ALFRED KAI-IN. 

